This invention relates to dredgers for removing sand, silt and like material from the river or sea bed and has application, for example to clearing wrecks, and providing trenches in which, for example, pipelines may be laid.
A suction dredger is the most widely used apparatus for removing such material, suction being created by a motor and pump unit, somewhat like a vacuum cleaner. However, if used for clearing wrecks, such apparatus has the disadvantage that small and/or lightweight articles from the wreck can also be lifted and, even if a screen is provided in the suction path, the articles may be small enough to pass through the screen, or be difficult to extract from the other debris lifted.
A modified form of the suction method, which is used in tidal waters, is to provide a vertical length of pipe above the area to be cleared near the lower end of which air is fed under pressure to pass upwardly through said pipe. This creates a vacuum, which will act to lift the sand, silt and like material and set it in suspension with the water, whereafter it may be carried away from the area by the tide. This method is reliable in reducing the possibility of small/lightweight articles being lost, but is time consuming due to the relatively small diameter of the pipe, normally around 0.5 meters, and hence restricted area covered.
Another method, which can be used in relatively shallow tidal waters, e.g. up to about 10 meters in depth, comprises mooring a tug, ship or other vessel in a fixed position above the area to be cleared and deflecting the propeller wash downwardly using a suitable guide plate. The wash disturbs the material around the wreck, which material is thereby lifted, set in suspension and carried away from the area by the tide. Apart from the shallow depth, another restriction of this method is that, for a large wreck, the position of the vessel must be changed progressively to cover the complete area of the wreck, which is difficult and time consuming.
EP-A-0328198 describes a method of dredging in flowing water comprising lowering a casing of a wing shape downwardly towards the area to be cleared, the casing carrying thrust means arranged so that the thrust means is directed downwardly, the orientation of the wing casing being adjusted in the water so that it presents a surface relative to the flow which causes a resultant downward vertical component of force to counteract the upward force provided by the thrust means, the thrust means directing a wash of water towards the areas to be cleared so that the turbulence created clears the sand, silt or like material covering the area.
This method of dredging is particularly useful for providing a trench across the sea bed. The wing shape casing is slowly towed along a line above the sea bed and the thrust means, which is directed vertically downwards, excavates a trench in the sea bed of a width which depends upon the material of the sea bed, its altitude above the sea bed, the power in the thrusters, its speed over the sea bed, and its pitch angle. In a typical example, the width of trench formed will be of the same order as the width of the wing shape casing.
Such a dredger, which is commonly known as a xe2x80x9cwing dredgerxe2x80x9d has been successful in producing a trench of a width sufficient to take a pipeline or, alternatively, to flatten an area of sea bed in preparation for works on the sea bed.
Reference is also made to EP-0419484 and GB 2315787 which describe wing dredgers is further detail.
The wing dredger is normally suspended below the support vessel by means of cables. One of the difficulties which has been found in practice with such an arrangement is that because of its relatively large surface area, the wing dredger will remain at a given depth, and the support vessel will of course rise and fall on the waves on the surface. This can cause unacceptable tension in the cables from the support vessel to the wing dredger and on the mounting means on the support vessel and the wing dredger, and a particular problem arises as the wing dredger is lifted towards the surface and is to be lifted out of the water onto the support vessel because, for example, the cable length by that time is considerable reduced and yet the support vessel is still moving up and down on the waves and the wing dredger is tending not to do so.
To a certain extent this problem can be overcome by providing in the lifting mechanism or in the cables a so called xe2x80x9cheave compensatorxe2x80x9d. Nevertheless, we have found this does not always operate quickly enough, especially with high waves. Heave compensators tend to be expensive and the amount of motion they can take into account is limited.
In seeking to address these problems and to provide a dredger which does not require to be slung from a boat upon the surface, the present invention has been devised.
In its broadest sense, the present invention provides a dredging apparatus comprising a body mounting first thrust means to direct, in use, a wash of water downwards towards an area of seabed or the like. The apparatus includes further thrust means to maintain the body of the apparatus above the seabed and to propel the body through the water. The body is in the form of a wing comprising a casing having ballast tanks to adjust to its submerged weight.
Preferably the further thrust means are arranged so that, in combination, they act as an attitude adjusting means to selectively adjust the attitude of the apparatus in a side to side (roll) orientation; and, independently, in a front to rear (pitch) orientation.
Preferably, to assist in taking the body down towards the seabed, the body will include means adapted to carry solid ballast, such as concrete blocks or iron chains, which can be jettisoned upon completion of a job to enable the dredger to return to the surface.
Conveniently, the casing is provided with an angled face at least along one (leading) edge thereof which at least in part, causes the resultant downward force component in use; this component can be varied by appropriately tilting the casing so that its upper surface is angled to the horizontal.
The first thrust means may comprise one or more propellers, each mounted within an open or bore, to rotate substantially parallel to the plane of the casing, in which case drive means for the propeller(s) are mounted on the casing. Means may be provided so that the direction of the jet streams can, separately or severally, be caused to flow inwards and outwards, as well as to the front and rear. In this way the jets may be set inwards and outwards with reference to the wing""s centre point through a measured circumference of 360 degrees with the plane of the propellers rotating at an angle of typically no more than about 40-45 degrees to the horizontal.
The further thrust means preferably also comprise one or more propellers driven by respective drive means.
In one embodiment, the drive means derive their power from an on-board engine, typically a diesel engine. Typically, the drive means comprise electric motors and the power is supplied by means of a diesel-electric power plant. Alternatively, the engine may operate a hydraulic pump, circulating pressurised fluid through the drive unit via flexible hoses, the drive unit comprising an hydraulic motor including gearing which meshes suitably with gearing on the or each propeller shaft.
Preferably, the engine receives a supply of air from a supply of compressed oxygen stored in suitable cylinders in the body of the apparatus itself. Preferably, carbon dioxide is stripped from the exhaust gases and discharged and the remaining nitrogen rich air is replenished with oxygen from the compressed oxygen supply and returned to re-aspirate the engine.
Alternatively, the engine receives an air supply and discharges its exhaust gases by means of a snorkel arrangement to the surface.
In a second embodiment, power is supplied from an external source. Preferably this is an engine, typically a diesel engine, housed within a suitably protected buoy floating on the surface of the water. The engine powers an electrical generator or hydraulic pump, the output from which is transmitted to the drive means in the body of the apparatus by means of an umbilical cord. In a modification of this embodiment, the power is supplied by means of an umbilical cord from a submarine or semi-submersible travelling above the apparatus.
The body of the apparatus will typically also house a number of sensors and scanning instruments. These will detect the orientation of the body, its heading, height above the seabed, the geography and geology of the seabed etc. These instruments and the control systems for the various thrust means all clearly require communication with the operators of the apparatus on the surface in a support vessel. This may be achieved by means of radio signals. An aerial lead from the body of the apparatus communicates with an aerial mounted upon a buoy floating on the surface. From there, signals are transmitted to and from the support vessel. Clearly, where the apparatus includes a snorkel or the engine is mounted within a buoy on the surface, suitably the aerial will be mounted on the same buoy.
However, preferably, the motion of the dredging apparatus and its on-board sensors and instruments is controlled from the support vehicle (ship on the surface, submarine, submersible or a remotely operated vehicle) by means of multi-channel sonar, each channel controlling a specific motion thruster or item of equipment.